Inductors are often used in conjunction with transformers in integrated circuits. A problem with such circuits is that inductors and transformers magnetically couple with each other. The resulting currents induced in the components can cause unwanted changes in their behavioural characteristics. To mitigate this problem, integrated circuits are often designed such that inductors and transformers are physically separated as far as is practical. However, inductors and transformers each occupy a large area on chip and it is desirable to minimise the chip area required for an integrated circuit. Furthermore, it is desirable to conserve chip area without compromising the performance of the integrated circuit.
It has been proposed to reduce the area required by a circuit comprising an inductor and a transformer by placing the inductor inside the transformer. Such a design is illustrated on FIG. 1. The inductor 101 comprises a loop which is enclosed by transformer 102. This design is problematic because the magnetic coupling between the inductor and transformer is sufficiently strong to cause either component to positively feedback a frequency generated in the other, thereby leading to a sustained oscillation. Such an unwanted oscillation can severely disrupt the operation of the integrated circuit.
Attempts to reduce unwanted oscillations associated with the design of FIG. 1 include reducing the coupling between the transformer and inductor by: reducing the size of the inductor relative to the transformer; and flipping the phase of the inductor relative to the transformer. Despite such attempts, unwanted oscillations as a result of coupling between the transformer and inductor remain a problem.
There is thus a need for an improved inductor-transformer structure which achieves both a reduction in the chip area occupied by the inductor and transformer, and mutual isolation of the inductor and transformer from each other by a further reduction in the magnetic coupling between the two.